1. Field of the Invention
This invention relates to optical switches, and more particularly to optical switches in which switching is accomplished by heating an interguide region between two optical waveguide cores to alter its refractive index.
2. Description of the Related Art
Optical switching between two waveguides has been accomplished in the past by altering the index of refraction of the guiding material at some critical point within the switch. In a directional coupler switch, which consists of two parallel waveguides sufficiently close to one another to exhibit evanescent coupling, one can either change the rate at which power is transferred from one guide to the other by changing the index of refraction in the interguide region (thereby changing the evanescent coupling), or one can frustrate the coherent power transfer process completely by appropriately changing the index of one guide relative to the other. (The first approach is called "delta-kappa" switching, the second "delta-beta"). A third approach, called "intersectional" switching, uses a reflective barrier installed at the crossing point of two intersecting waveguides. Small changes in the refractive index of this barrier either allow the light to pass through the barrier and into the first waveguide, or cause it to undergo total internal reflection into the second.
Various mechanisms have been used to control the refractive index of the guiding material. One approach uses the electro-optic effect to vary the refractive index by applying an electric field across the guiding region, normally by means of a reverse-biased Schottky diode. Another approach relies upon the injection of free-carriers into the region. While useful switching rates have been achieved with these techniques, the electro-optic approach is dependent upon the polarization of the beam. The free-carrier injection approach, although polarization independent, suffers from high losses due to carrier absorption.
One can also change the index of refraction by heating the guiding material. This has been done by putting a metal strip of film over the critical region; current is passed through the strip to heat it, and the heat spreads into the underlying material, raising the temperature locally. Because of the finite thermal coefficient of the index of refraction of the guiding material, a localized change in the index results.
A thermal transfer technique for optical switching is disclosed in Haruna and Koyama, "Thermo-Optic Effect in LiNbO.sub.3 for Light Deflection and Switching", Electronics Letters, Vol. 17, No. 22, Oct. 29, 1981, pp. 842-844. In this demonstration a layer of NiCr was vacuum evaporated as a strip heater on a LiNbO.sub.3 crystal. When a 60 Hz AC voltage was applied to the strip heater, a laser beam directed through the crystal was deflected. Application of this approach to light switching as well as optical deflection was demonstrated by placing a small heater on a waveguide to provide a partial index change. Input and output channel waveguides were connected by a bridge waveguide that was covered directly by a NiCr film heater with a resistance of 790 ohms. The application of appropriate voltages to the film heater resulted in a thermo-optically induced waveguide which bridged the input and output channel waveguides. In the absence of the applied voltage to the film heater, the incident guided modes leaked to the substrate.
In Haruna and Koyama "Thermo Optic Deflection and Switching in Glass", Applied Optics, Vol. 21, No. 19, Oct. 1, 1982, pp. 3461-3465, a NiCr film heater was vacuum evaporated on a glass substrate. The film heater was supplied by a pulsed voltage with a 200-Hz repetition rate and a 2-msec pulse width in one instance, and a 40-Hz repetition rate and 5-msec pulse width in another instance; titanium-sputtered film heaters were used for the second case. In the first test the heater was used to control optical switching between an output waveguide and the substrate, while in the second test the heater was used to control optical switching between two output waveguides.
More recently, beam deflection within a planar waveguide was achieved with a silver stripe heater having a resistance of 73.4 ohms evaporated upon a polymethyl methacrylate buffer layer on a polyurethane waveguide structure. Switching times of about 10-ms were measured, which the authors believed could be reduced to a few milliseconds with optimized thermal design and regulated power dissipation in the heater. N. B. J. Diemeer et al., "Polymeric Optical Waveguide Switch Using the Thermo-Optic Effect", Journal of Lightwave Technology, Vol. 7, No. 3, March, 1989, pages 449-453.
This application describes deflection of a light beam within a slab, and not the switching of light between waveguides, so that it is not a pure waveguide switch.
Unlike the electro-optic switches, the thermal switches discussed above have the advantages of operating independent of the input beam polarization, and being applicable to relatively thick (on the order of 3-5 microns) waveguides. However, their switching speeds are quite slow. The Haruna/Koyama devices were found to have a switching speed on the order of 1 ms, while the Diemeer device if optimized was expected to have a switching speed of "a few" ms. These relatively slow responses correspond to switching rates of not more than about 1 KHz, which is too limited for most applications.